Magnetic Extensometer

Magnetic Extensometer

More Information

GEO-DS-0041 Magnetic Extensometer

4700 Borehole Magnetic Extensometer Manual

Australia Tel: +61 (0)3 8720 6700
Indonesia Tel: +62 21 3040 4422
New Zealand Tel: +64 (0)7 850 2610
South Africa Tel: +27 (0)10 900 2055


The HMA Geotechnical magnetic extensometer (4700 Series) monitors changing subterranean displacements at various horizons within a borehole.

The measurement of soil and rock movements including settlements, heave and lateral movements in foundations and embankments. Also used in subsidence, movement of natural and cut slopes, quarry and mining excavations, displacement of retaining walls, bridge piers and butments and relaxation of rock around tunnels.

• Reliable, accurate, and simple to install and read
• No limit to the number of points that can be installed in one borehole, with little extra cost
• Can be extended simply
• One probe required for many installations
• Very economical

Ring magnets sliding on a central access tube and within a collapsible outer tube are fixed in the ground (via a borehole) at locations where movement is to be monitored. These magnets may be installed in a Ø100 mm borehole, or placed subsequently during the construction of earth works. Location of the targets is accomplished by passing a Magnetic Extensometer Readout Probe (Model 9200) through the access tube. When the probe enters the magnetic field generated by the target, an audible signal is emitted at ground level. Measurements made with a magnetic readout probe may then be related to any convenient datum. The targets are designed to move independently of the access tube and within the protection of the outer collapsible tube.

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